As an electrophotographic color printer, there is know a so-called tandem type color printer in which photosensitive drums corresponding to respective colors of yellow, magenta, cyan, and black are provided in parallel with each other. A color printer of this tandem type is configured such that four photosensitive drums are detachably mounted in a main body casing as a single unit.
This color printer includes drum subunits respectively holding the photosensitive drums, and a pair of side plates supporting the drum subunits from both sides in a width direction of the photosensitive drums. A developing cartridge including developing roller is detachably mounted between the side plates so as to correspond to each churn subunit. The drum subunits, the developing cartridges, and the pair of side plates are detachably mounted in the main body casing as a drum unit.
Both end portions of a drum shaft of each of the photosensitive drums protrude outward from both side surfaces of the corresponding drum subunit. Each side plate is formed with four axial holes at a regular interval in the arrangement direction of the photosensitive drums. The end portions of the drum shaft are respectively inserted into the axial holes, and each end portion of the drum shaft is pressed by a wire spring to an inner surface of the axial hole so as to position the drum shaft (photosensitive drum).
In the above-described color printer, if stress is applied to the photosensitive drum, the drum shaft could be deviated against an urging force of the wire spring. As a result, positioning precision of the photosensitive drums deteriorates, so that quality of an image formed by the photosensitive drums becomes worse.
Further, if the surfaces of the photosensitive drums are damaged or foreign matter adheres thereto, the images formed on the sheet are disordered. Therefore, if the surface of any photosensitive drum is damaged or foreign matter adheres thereto in the assembly or use of the color printer, the photosensitive drum is required to be replaced.
However, in the above-described color printer, both end portions of the drum shaft of each of the photosensitive drums are inserted into the pair of side plates. If one of the photosensitive drums needs to be replaced after the drum unit is assembled, the photosensitive drum cannot be independently removed from between the pair of side plates. That is, the pair of side plates has to be detached from all the drum subunits to remove the drum subunits therefrom. Therefore, it takes a lot of time to replace the photosensitive drum. Then, after the photosensitive drum is replaced, one side plate has to be attached to the four drum subunits, so it takes a lot of time to reassemble the color printer.